This invention relates to an improved large chemical reaction vessel useful for conducting exothermic reactions under conditions of highly elevated pressure and wherein agitation conditions within the reactor strongly affect the quality or quantity of the finished product and also where the smooth condition of the inside surface of the reaction vessel is important for preventing buildup and fouling of that surface. The suspension polymerization of vinyl chloride is a particularly good example of such a reaction and will be used to exemplify the use of the improved reactor and operating conditions.
Vinyl chloride monomer is polymerized in a water suspension under high pressure and the polymerization reaction releases a considerable amount of heat. Unless this heat of polymerization is removed from the reacting mixture, the heat will raise the temperature of the continuing polymerization. Raising the temperature of the polymerization in turn increases the rate of the polymerization reaction resulting in an even greater rate of heat release and a possible run-away reaction and the subsequent discharge of the reactor contents via emergency relief methods. Thus, temperature control is an absolute requirement for suspension vinyl chloride polymerizations. It is also necessary to subject the suspension polymerization to a relatively high level of agitation. The high agitation level is designed to accomplish many things. The high agitation promotes the transfer of the heat of reaction to the wall where it can be removed by conduction through the wall. The high agitation also keeps the contents of the reactor well mixed and prevents the segregation of the vinyl chloride monomer into regions of low agitation which results in "hot spots" and the possibility of reactor over pressurization. The high agitation also plays a critical role in the particle size, particle size distribution, and other key properties of the finished polyvinyl chloride product. Because the polymerization is run at elevated pressure, the pressure-retaining wall of the reaction vessel must be relatively thick. The increased thickness becomes an added barrier to efficient heat transfer through this thick wall. As the industry has reduced capital costs by building larger polymerization vessels, in the size range from 10,000 gallons to 60,000 gallons or more, the walls of the vessels have perforce been made thicker to safely contain the internal pressures of up to 225 pounds per square inch or more created by the vapor pressure of the vinyl chloride. These thick walls result in reduced ability to remove the heat of polymerization by the standard method of heat conduction through the walls. In addition to this thick-wall effect, the amount of wall surface area per unit volume of the reactors is reduced as the size of the reactor is increased, thus compounding the problem.
There have been several methods described in the prior art for accomplishing the necessary increased heat removal. One method is to use reflux condensers atop the reactors to condense vaporized vinyl chloride, cool it, and return the cooled liquid to the reactor. Reflux condensers are efficient at removing the heat of polymerization but their use introduces some new requirements. They require that the liquid level in the reactor be kept low to allow for the control of foaming and they often require the use of chemical antifoam additives that are costly and sometimes adversely affect the quality of the product. Also, the use of too much reflux cooling can have a deleterious effect on the quality of some grades of product poly vinyl chloride resin, thus, limiting the utility of this method.
A second method for heat removal that has been described is to place cooling structures such a baffles, coils, and the like inside the polymerization vessel. The amount of heat that can be removed by this means is also limited because baffles or coils will alter the highly critical agitation within the reaction vessel and interfere with the quality of the product if excessively or incorrectly used.
A third heat removal method is the use of a reaction vessel having a thin inner wall with a jacket between the inner wall and thick outer wall as taught by Perryman in U.S. Pat. No. 5,027,971. In the teachings of this patent, the reactor is formed by welding flow channel walls to the inside of a thick pressure supporting cuter wall and then subsequently plug welding the thin inner wall to the flow channel walls to produce a reactor with a cooling jacket between the walls. The reactor as described in U.S. Pat. No. 5,027,971 has several disadvantages. The plug weld points are internal defects on the surface in contact with the polymerization mixture and, as such, can be locations where polymer buildup and fouling can occur. Substantial fouling requires that the reactor be shut down, opened, and cleaned to remove the fouling deposits. The fabrication cost of the Perryman reactor is believed to be high because of the complex welding required.